A New Pattern of Phytomelanin Deposition in Cypselae of Bahieae (Asteraceae, Heliantheae Alliance)

Phytomelanin is the dark and highly inert substance found in the fruit walls of the Heliantheae alliance clade within Asteroideae (Asteraceae). Phytomelanin deposition pattern in the cypselar wall was studied in two taxa of the Bahieae—Bahia pedata A. Gray and Palafoxia arida B. L. Turner. The pericarp anatomy studies have revealed that the phytomelanin deposits in the schizogenous space between the hypodermis and fiber layer. Our findings reveal that phytomelanin deposition is associated with fiber and has also brought out a new pattern of phytomelanin deposition between inner (fibrous zone) and outer sclerified layers (hypodermal) in the schizogenous space. The study recorded a first-time report of a second layer of phytomelanin outer to the sclerified hypodermal cells at maturity. Our observations point to phytomelanin precursors being synthesized as pliable fluid substances associated with fiber cells and solidifies later in the intercellular space. The study also recorded the first report for the occurrence of tracheoidal cells in the pericarp wall of the Bahieae within the Heliantheae alliance. Our study has provided insights to decipher evolutionary patterns and the significance of phytomelanin in the phytomelanic fruit clade (PFC), and it could help to resolve long-standing debates on deposition patterns and the site of synthesis of phytomelanin.